运行环境：asp.net 4.5.2。

当我们向GlobalConfiguration.Configuration.MessageHandlers添加一个DelegatingHandler派生类后，很容易发生即使命中了Action，但方法参数值为null的问题。

在大多数情况下，我们会在DelegatingHandler派生类里，重写async Task<HttPResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)方法。

在方法内，做一些事情，比如说访问日志记录，任意校验，添加Header，修改URL(重写)等工作。

其中最重要需要注意的一点在于request.Content，当我们在方法内访问了request.Content (get)之后，而不对request.Content进行赋值(set)的话，会发生什么呢？

这会导致我们的方法(action)无法获取到客户端Post上来的数据，导致方法参数值为null。

这是为什么呢，这个中原因，我没去深究。

现在附上解决代码：

 1 public class DefaultHandler : DelegatingHandler
 2 {
 3     protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
 4     {
 5         request.RequestUri = new Uri(request.RequestUri.ToString());
 6 
 7         MediaTypeHeaderValue contentType = request.Content.Headers.ContentType;
 8 
 9         if (contentType != null)
10         {
11             switch (contentType.MediaType)
12             {
13                 case "application/x-www-form-urlencoded":
14                     {
15                         NameValueCollection formData = await request.Content.ReadAsFormDataAsync(cancellationToken);
16                         request.Content = new FormUrlEncodedContent(Correct(formData));
17                         //TODO:在这里对formData进行业务处理
18                     }
19                     break;
20                 case "multipart/form-data":
21                     {
22                         NameValueCollection formData = await request.Content.ReadAsFormDataAsync(cancellationToken);
23                         request.Content = new FormUrlEncodedContent(Correct(formData));
24                         //TODO:在这里对formData进行业务处理
25                     }
26                     break;
27                 case "application/json":
28                     {
29                         HttpContentHeaders oldHeaders = request.Content.Headers;
30                         string formData = await request.Content.ReadAsStringAsync();
31                         request.Content = new StringContent(formData);
32                         ReplaceHeaders(request.Content.Headers, oldHeaders);
33                         //TODO:在这里对formData进行业务处理
34                     }
35                     break;
36                 default:
37                     throw new Exception("Implement It!");
38             }
39         }
40 
41         return await base.SendAsync(request, cancellationToken);
42     }
43 
44     private static IEnumerable<KeyValuePair<string, string>> Correct(NameValueCollection formData)
45     {
46         return formData.Keys.Cast<string>().Select(key => new KeyValuePair<string, string>(key, formData[key])).ToList();
47     }
48 
49     private static void ReplaceHeaders(HttpHeaders currentHeaders, HttpHeaders oldHeaders)
50     {
51         currentHeaders.Clear();
52         foreach (var item in oldHeaders)
53             currentHeaders.Add(item.Key, item.Value);
54     }
55 }

在Global.asax添加代码：

1 protected void Application_Start()
2 {
3     GlobalConfiguration.Configure(WebApiConfig.Register);
4     GlobalConfiguration.Configuration.MessageHandlers.Add(new DefaultHandler());
5 }

模型类：

 1 public class TestModel
 2 {
 3     [JsonProperty(PropertyName ="I")]
 4     public long Id { get; set; }
 5 
 6     [JsonProperty(PropertyName = "N")]
 7     public string Name { get; set; }
 8 
 9     [JsonProperty(PropertyName = "M")]
10     public decimal Money { get; set; }
11 
12     [JsonProperty(PropertyName = "IE")]
13     public bool IsEnable { get; set; }
14 
15     [JsonProperty(PropertyName = "CD")]
16     public DateTime CreateDate { get; set; }
17 
18     [JsonProperty(PropertyName = "UD")]
19     public DateTime? UpdateDate { get; set; }
20 }

ApiController：

1 public class DefaultController : ApiController
2 {
3     [HttpPost]
4     public TestModel Find(TestModel model)
5     {
6         return model;
7     }
8 }

Request Body：

{"I":10000,"N":"TestModel","M":21547855.0001,"IE":true,"CD":"2015-12-10 12:12:12","UD":"2016-01-01 01:01:01"}

Fiddler4测试：

测试结果：

解决办法来自：http://stackoverflow.com/questions/27333419/modify-request-content-in-webapi-delegatinghandler